2 Stage GM-70 amp in A2... possible? - diyAudio
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Old 18th December 2012, 02:11 AM   #1
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Default 2 Stage GM-70 amp in A2... possible?

Thinking of building a GM-70 amplifier in Single Ended.

Was wondering if a 2 stage amplifier would be able to achieve A2. Any ideas?

If yes, what topology and what driver tube would be suitable.
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Old 18th December 2012, 02:13 AM   #2
cotdt is offline cotdt  United States
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Why A2? Just turn up the voltage.

D3A -> GM70
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Old 18th December 2012, 02:42 AM   #3
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How does turning up the voltage drive it into A2?
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Old 18th December 2012, 03:01 AM   #4
cotdt is offline cotdt  United States
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That's not what I said. If you want more power, simply increase the voltage in A1. In A2, the load from the GM70 is really low maybe 1k, so you'll need a 300B to drive it, which means 3 stages. Even then, I've never tried it, it might not even work.
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Old 18th December 2012, 04:18 AM   #5
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Meaning not quite possible to do A2 with 2 stages. ok.
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Old 18th December 2012, 04:22 AM   #6
cotdt is offline cotdt  United States
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I never said it was not possible. I only said that it was pointless, because you can simply give it more voltage in A1.

You can try a D3A -> MOSFET -> GM70, but it may or may not work.

Why are you so obsessed with A2? To get worse sound quality while gaining nothing?

Last edited by cotdt; 18th December 2012 at 04:24 AM.
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Old 18th December 2012, 05:03 AM   #7
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Thanks for the replies.

As for the assertion that I am obsessed with A2, please refer to my first post. I just wanted to know if it could be done. The answers thus far seem to point to the fact that my original hopes may not be quite possible.

Kind regards.
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Old 18th December 2012, 12:44 PM   #8
mwiebe is offline mwiebe  United States
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There is a thread somewhere here by Andrea Ciuffoli on two-stage 845 SET which will get you partly there. Depending on how much grid current you want to push.
Matt

845 Amplifier with only two stages
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Old 18th December 2012, 02:38 PM   #9
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"Was wondering if a 2 stage amplifier would be able to achieve A2."

What input sensitivity are you aiming for? If you have a preamp capable of ten volts output, maybe; otherwise probably not. But you need to start by defining how much voltage you need to drive the GM70 into A2 and then work backwards to the input. In other words if you need 150VRMS drive (to pick a random number!), how will you get from your input voltage (that you haven't defined) to 150VRMS in one stage?
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Old 18th December 2012, 04:19 PM   #10
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It's very difficult. To me it's almost impossible. Driving GM70 to A2 requires low impedance driver that can make 300Vpp. From my experience, tubes with Rp > 1.5K cannot push power tube's grid positive. I once tried E810F/7788 and found it's having hard time to push 211 to A2. GM70 is even more difficult, you can see the picture.
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